Behavior of volcanic ash–soil mixtures under one-dimensional compression testing

Volcanic ashes (VA) are one of the by-products of explosive volcanic eruptions. They can be used as a soil stabilizer due to their cementitious properties as an eco-friendly soil stabilization approach. In this study, the impact of VA as an additive material (up to 20%) was investigated on the behavior of a clayey soil under one-dimensional compression tests and uniaxial compression tests. To this aim, the VA percentage effect, curing conditions, i.e. the optimum moisture content (OMC) and saturated sample, and curing time, on the oedometer modulus, and the uniaxial compression strength (UCS) are investigated. Results show that the addition of VA increases the UCS continuously in saturated conditions. However, this improvement is considerable for 5% additional VA at the OMC state and it induces 325% improvement in UCS. The maximum improvement of UCS occurs at 20% addition of VA in saturated condition. It was also revealed that VA-soil mixtures are more sustainable at low stress levels and the oedometer modulus increases with the VA addition. A long-term curing time leads to an increase of the fabricated bonds due to the pozzolanic reaction. Additional VA has no significant effect on the consolidation parameters specifically for short-term curing time.


Abbreviations
www.nature.com/scientificreports/ modeling. Also, the consolidation parameters namely the compression index (C c ), swelling coefficient (C s ), and recompression index (C r ) for the saturated conditions are measured for the VA-soil mixture at different curing times. Last but not least, the oedometer elasticity modulus of the VA-soil mixture is determined at different stress levels, from 25 to 400 kPa. This parameter is the most important one for the settlements estimation. To this aim, several standard tests namely, proctor tests, one-dimensional compression, and uniaxial compression tests were conducted. Four proportions of VA, 5%, 10%, 15%, and 20%, are mixed with clayey soils. The samples were prepared and cured at the OMC and saturated conditions representing the in-site project limitations namely weather condition. Uniaxial Compression Strength (UCS) at both OMC and saturated conditions are determined. Also, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were used to show the bond formations before and after treatment.

Materials
Clayey soil. The soil used in this research was extracted from the center of Mashhad-Iran. The soil physical and mechanical properties were determined according to the American Society for Testing and Materials (ASTM) [41][42][43] . These properties are presented in Table 1. The soil is classified as CL-ML as per Unified Soil Classification System (USCS) 44 and as A-6 according to the AASHTO classification standard (ASTM D3282) 45 . Figure 1 shows the soil grain size distribution curve. Clay (particle size < 0.002 mm) 15.5

Consistency limit
Liquid limit 26.0  www.nature.com/scientificreports/ Volcanic ash (VA). The pozzolan used in this research was extracted from a natural pozzolan mine located at 140 km northwest of Mashhad in Iran. The coarser particles were passed throughout a crusher machine and a #200 sieve to assure that the VAs particle sizes are fine enough. The specific gravity (G s ) of the VA is 2.05 based on the ASTM D854-14 43 . It is classified as a N pozzolanic material according to ASTM C618 46 standard. Table 3 describes the chemical compounds and elements of the VA estimated according to the X-Ray Diffraction (XRD) technique. Figure 2 illustrates the soil and VA used in this study. Figure 3 shows the Scanning Electron Microscope (SEM) photos of the soil and VA. In terms of grain size and shape, the soil sample ( Fig. 3a) has larger and smoother particles compared to VA (Fig. 3b). In fact, due to the shattering process, VA has angular particle shapes.

Experimental program
A series of standard tests including compaction, uniaxial compression, and one-dimensional compression tests were conducted to investigate the VA addition effect on the soil improvement performances. These tests were carried out considering various curing times and percentages of VA. It is worth noting that compaction and uniaxial tests were performed for validation and control of the improvement procedure. In order to study the oedometric parameters and consolidation parameters, one-dimensional compression tests were conducted. A period 28 days was considered for the curing time in the uniaxial compression tests. In terms of one-dimensional compression tests, Table 4 summarizes the varied parameters and the testing program. The general designation of each test is named VA n D t -X in which, VA n indicates the n VA-soil mixture percentage, D t the t curing time days, and X the curing conditions. VA 0 , VA 5 , VA 10 , VA 15 , and VA 20 respectively stand for 0, 5, 10, 15, 20% of VA-soil mixtures. These mixtures are based on previous investigations 31 . The curing procedure is done at the Optimum Moisture Content (-OMC) or in Saturated (-S) conditions in 7, 14, 28, and 90 days. For instance, VA 5 D 7 -S represents 5% of VA in 7 curing days considering saturated conditions. It should be noted that all the tests were performed at a relative standard proctor compaction (RC) of 90%.

Sample preparation and testing procedure
The dried soil was mixed with the desired percentage of VA in dry conditions. A homogenous mix of the VAsoil was prepared by adding water and then mixed vigorously. The admixture was then wrapped in plastic bags and continuously mixed by shaking. Finally, the bags were stored until conducting the test. Considering ASTM D698 47 , the standard proctor compaction test was carried out to determine the OMC and Maximum Dry Density (MDD) of the VA-soil mixture (different VA contents). In these tests, no curing time was considered, and they were immediately carried out after preparation. Cylindrical specimens with 50 mm of diameter and a height of 100 mm were utilized for determining the Uniaxial Compression Strength (UCS) following the ASTM D2166 48 . It is worth noting that remolded samples were prepared with OMC based on the moist-tamped method 49 . All prepared specimens were again wrapped into plastic bags and maintained for several days inside a curing chamber with a controlled temperature of 23 ± 2 °C. In terms of saturated curing conditions, the samples were drowned into a pot full of water. A displacement control loading was applied with a rate of 1 mm/min for the  www.nature.com/scientificreports/ uniaxial tests. This loading rate value corresponds to the deformation rate which is created beneath the pavement subgrades due to the traffic loads 38 . One-dimensional compression tests were carried out on the stabilized soils regarding ASTM D2435 50 . A cylindrical mold of 75 mm in diameter and 20 mm in height was used for these tests. The specimen preparation was similar to the uniaxial compression tests, but the samples were placed in a curing chamber for 7, 14, 28, and 90 days. For each percentage of VA, two samples were reconstituted, i.e. for saturated and OMC testing conditions. Moreover, in terms of saturated condition, after assembling the porous stones on the top and bottom of the sample, the pot of the one-dimensional compression apparatus was brimmed with water and the temperature of the testing room kept constant at 23 ± 2 °C (see Fig. 4). The stress level in this tests is limited to 400 kPa which corresponds to secondary roads 37 .

Results and discussions
Effect of VA on OMC and MDD. Figure 5a illustrates the dry density variation with the moisture content for different VA percentages. Figure 5b shows the results of the conducted standard proctor tests. The VA percentage impact on OMC and MDD is presented. By increasing the VA-percentage, from 0 to 20%, the MDD decreases approximately 9% (from 1920 to 1750 kg/m 3 ). This trend can be attributed to the VA specific gravity and the grain size distribution of the mixture which was also observed by Hossain and Mol 38 .
At first, additional VA powder coats the soil particles. It leads to coarser particles admixture and the free space volume increases consequently. However, the free space (void ratio) enlarges until the VA coats all particles surface. Then, more VA addition to the soil will fill free spaces, and the void ratio decreases 38 . The impact of adding more VA to the soil will then become low. This behavior can also be delineated by Fig. 6 which reveals the void ratio variation with the VA percentage. By increasing VA percentage, the void ratio increases of 10% and tends to decrease afterward. This reduction can be due to the VAs particle size. www.nature.com/scientificreports/ An adverse trend is observed for the optimum moisture content variation versus the VA percentage. The OMC increases up to 2% by adding 20% of VA and the amount of water is in direct relationship with the VA content. As addressed by different researchers like Hossain and Mol 38 . They mentioned that this trend can be attributed to the VA-soil admixture water absorption for pozzolanic reactions. Figure 7 illustrates the SEM photos for the VA 10 D 90 -S sample. Fine grains of VA cover the clay particles, and chemical bonds are fabricated. The chemical bonds contain CSH and CAH (bright spots in Fig. 7). An Energy Dispersive Spectroscopy (EDS) test was carried out to determine the compounds within the crystal bonds. As shown in Fig. 8, the amount of calcium found in CSH and CAH is high. Thus, the bright spots which are representing the solid bonds, are produced by the pozzolanic reaction 27 .
Effect of VA on UCS. Figure 9 demonstrates the effect of adding VA on the UCS after 28 curing days. The UCS is around 17 kPa, while it raises up to 147 kPa for stabilized soils with 20% of VA. It represents an improvement of 760% once it is cured considering a saturated state (Fig. 9a). The presence of VA in the soil induces resistance bonds which leads to the soil cohesion enhancement and higher UCS.
On the other hand, UCS considering the OMC curing conditions increases as well, however, the largest strength appeared for VA 5 . Indeed, by adding 5% of VA to the soil, UCS starts from 45 kPa at VA 0 and sharply increases to 170 kPa (277% of improvement) for VA 5 . Then it decreases non-linearly when adding extra VA to the soil, to approximately 74 kPa (64% of improvement) for VA 20 (Fig. 9b). This trend was also been observed by 27 . The pozzolanic reactions cannot be completed due to the lack of water and this can make the situation worse by adding extra VA which leads to a higher void ratio in the soil sample. Moreover, VA particles are naturally non-cohesive, thus, by adding extra VA, the UCS decreases due to the absence of bonds.
Oedometer modulus. A total of 36 1-D compression tests were conducted to investigate the VA addition effect on the oedometer modulus. To this aim, the secant elasticity modulus was determined for five stress levels,   In this study, a parameter is named IF, Improvement Factor. It can be obtained using the following equation: (1)  Effect of the VA percentage on IF. Figure 11 shows the VA percentages variation versus IF for the OMC condition. Increasing the VA percentage until 15%, the IF increases for all the stress levels and declines for VA 20 . Indeed, for a higher amount of VA, the higher settlement is induced by VA 20, and IF reduces consequently. Figure 12 shows the results of the IF for the saturated one-dimensional compression tests (conventional consolidation tests). The more VA added to the soil, the more IF increases. Indeed, settlement decreases consistently with increasing the VA percentage for the saturated conditions. By comparing the results for both OMC and saturated conditions, the IF evolution can be attributed to the lack of water in the VA 20 . Hence, fabricated resistant bonds are smaller as in VA 15 . For the saturated condition, since the sample is brimmed with water, the VA pozzolanic reaction continues, and a higher IF is reached. The VA compressibility properties are higher than the soil one 30 ; therefore, for unsaturated conditions, adding more VA from 15 to 20% requires more water for the pozzolanic reaction. Settlements are reduced for VA 20 due to the existence of more VA in the sample.
Effect of the curing time on IF. As already stated, VA is a type of cementitious material, so the curing time (pozzolanic reaction period) has a remarkable impact on the samples strength. In the current study, the oedometer  Stress level effect on IF. As stated previously, the oedometer modulus is determined for five different stress levels. Figure 15 a,b indicate the IF changes with various stress levels for VA 15 in OMC and saturated conditions. An indirect nonlinear relationship between the stress level and IF is observed for all the curing times. The IF decreases dramatically with the stress level increase. The bonds between the soil and VA which were fabricated during curing time can be fractured under the loading application. Indeed, the higher the load level, the more bonds are broken. It can also be seen in Fig. 15 that the relationship between the IF and stress level can be interpolated as a power function, IF = k( σ P atm ) n , and the measures of the R-squared value indicate a good fit for the data. In the interpolated function, k and n are the constants which vary due to the curing condition, time, and VA content. These parameters, k and n, respectively vary based on the curing condition and stress level.
As indicated in Fig. 16, the variation of k and n with the percentage of VA shows an analogous trend with Figs. 11 and 12. Additional VA expands the fabricated bonds which make the VA-soil mixture more brittle and leads to the higher values of k except for VA 20 -OMC. Therefore, this fragility causes more cracks appearance www.nature.com/scientificreports/ as the consequence of the stress application. Also, k values are greater for the saturated conditions rather than OMC states due to the presence of more pozzolanic bonds. In fact, the key parameters that affects k, are the VA percentage and curing conditions (time and moisture) and it depends on the formation of bonds. n has negative values which indicates the reduction effect of additional VA for higher stress levels. Likewise, n values are lower for saturated conditions. Because the more the moisture content is, the less the clay behavior is brittle. Therefore, samples prepared using OMC conditions are more brittle than the saturated state, and n has a greater value for the OMC conditions.
Effect of VA on the consolidation parameter. Figure 17 shows the one-dimensional compression tests at saturated conditions (conventional consolidation tests). Figure 17 presents the typical consolidation test graph for different percentages of VA at 90 days of curing time. As stated previously, additional VA increases the void ratio which leads to shifting the graph up. The Compression Index (C c ) shows the capability of the soil to decrease its volume under external loads. Figure 18 illustrates the variation of C c with the VA percentage at different curing times. As seen, adding VA to the soil decreases the C c , while more VA influences C c for the short-term curing time. This effect becomes important when the sample is cured for 90 days. Shaped bonds are fragmented by additional VA for short-term curing time. This short-term curing period trend can be attributed to the fact that resistance bonds are broken at high stress levels and C c is determined at the final increment of loading (tangential slope at 400 kPa stress). In terms of 90 days curing time, for example, C c decreases down to 60%, from 0.082 to 0.032, for VA 20 -D 7 -S and VA 20 -D 90 -S. The swelling coefficient (C s ) which reveals the capability of the soil to increase its volume after unloading. Figure 19 shows changes of C s with the VA content. Correspondingly, VA has a negligible impact on C s and exclusively for short-term curing time. Nevertheless, C s decreases for long-term curing times with increasing the VA percentage. Pozzolanic bonds hinder swelling to a limited extent when they are well fabricated. More specifically, the Recompression Index (C c determination at the initial increment of loading, C r ), is also assessed to elaborate on the effect of additional VA on settlement properties. Figure 20 indicates the C r changes with VA

Conclusion
In this study, the feasibility of using Volcanic Ashes (VA) for clayey soils stabilization is investigated by employing compaction, uniaxial compression, and one-dimensional compression tests. Four different percentages of VA (i.e. 5, 10, 15, and 20%), were added to the soil in order to study the VA effect on the uniaxial strength as well as on the oedometer modulus at the Optimum Moisture Content (OMC) and fully saturated conditions during curing periods varying from 7 to 90 days. The impact of VA on consolidation parameters was evaluated. The laboratory investigations allow to obtain the following conclusions: www.nature.com/scientificreports/ • Using VA increases the void ratio and OMC. This can be attributed to the space and bond formation after the pozzolanic reaction. Therefore, additional VA increases the water absorption, • For the OMC conditions, the optimum percentage of additional VA is respectively equal to 5% and 15% for the uniaxial and 1-D compression tests. While, for saturated curing conditions, no optimum VA was observed for both tests. In other terms, it was found that the presence of water allows the production of the bonds formation, • The soil stiffness parameters are significantly improved by adding VA, and this improvement is more important for saturated curing conditions, • Concerning the curing periods, for a constant percentage of VA, short-term curing condition (7 days) increases the Improvement Factor (IF). Nevertheless, the enhancement is more remarkable for long-term curing conditions (14 days and more). Indeed, more bonds are fabricated by the pozzolanic reaction throughout the curing time, • In terms of stress level, it can be stated that using VA would be more productive for low service loads. Otherwise, the fabricated bonds fragment, hence, both stiffness and resilience are diminished. Therefore, stabilization with VA would be functional for pavement design in contrast with foundation-based improvement, • The addition of VA decreases the Recompression Index (C r ) more dramatically than the Compression Index (C c ). It has a low impact on the Swelling Index (C s ). Adding extra additional VA has a negligible effect on the consolidation parameters at short-term (up to 28 days) and it becomes considerable for long-term (i.e. 90 days).

Data availability
All data generated or analyzed during this study are included in this published article. The raw data is also available from the corresponding author on reasonable request.